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The underlying image quality is an important aspect of a medical ultrasound imaging scanner. Throughout the field, there is a constant effort to improve image quality. However, ultrasound imaging remains a modality that is very patient dependent. The ultrasonic imaging and detection of important pathological phenomena may fail if the image is not of sufficient quality to assure diagnosis.
In addition to image quality limitations, there are physiological effects that may impact ultrasonic imaging. For example, small variations between consecutive heartbeats may adversely impact imaging and image quality. These beat to beat variations may be caused by may factors including: respiration, heartrate variations, double systoles, early excitations and a number of other physiological effects. These variations present a limitation for performing both repeatable and reproducible measurements of ultrasound derived parameters such as blood flow velocities, wall motion or wall thickening, for example.
Because of the difficulties inherent in performing repeatable ultrasonic measurements, several measurement protocols have been developed wherein an ultrasonic imaging operator is instructed to repeat the same measurement for a certain number of consecutive heartbeats and report the average value. These measurement protocols improve the repeatability and reproducibility of the measurement because an average measurement is reported, rather than a single measurement which may not represent a normal heartbeat.
The prior art describes techniques for temporal processing of ultrasound data to reduce noise. These techniques include consecutive measurements at a given location which are combined in order to reduce noise. However, the problem with these techniques in cardiac imaging is that the imaged organ is moving. That is, as the heart beats, it changes its state in terms of blood flow pattern, cavity shapes, wall motion and wall thickness, for example. Because the imaged organ is moving and changing, simple temporal processing techniques are ineffective and the techniques of the prior art yield images of unacceptable image quality. The same limitations on imaging of the heart also apply to other organs and vessels that move or change. Many organs may change according to respiration or blood pulsation, for example. As a result, when using temporal processing, the dynamics and image quality of the imaged organ are degraded as a side effect of the temporal averaging.
The prior art also describes techniques for temporal processing of ultrasound data to increase frame rate. For example, U.S. Pat. No. 5,099,847 issued to Powers et al. describes a techniques for increasing the display frame rate of a medical ultrasound imaging system. The system generates a trigger signal based on a predetermined event in the subject""s cardiac cycle. The system produces a first series of image frames in response to a first trigger signal during a first cardiac cycle and a second series of image frames in response to a second trigger signal during a second cardiac cycle. The image frames of the first and second series are then interleaved in the order of occurrence in the cardiac cycle to produce a frame rate twice as great as the imaging frame rate. The Powers patent thus relies on frame recordings at different instants in the first and second cardiac cycles in order to obtain a merged sequence of images with a higher frame rate. However, while the Powers patent may provide a higher frame rate, the sequenced images are taken from different cardiac cycles. Because the images are taken from different cardiac cycles, one or more of the cycles may be affected by artifacts induced from, for example, respiration. Additionally, the subject""s position or the probe""s position may have moved from one cardiac cycle to another. Interleaving frames from cardiac sequences affected by motion may yield a jittery and inaccurate combined image sequence which may adversely affect diagnosis.
Thus, a need has long existed for an improved ultrasonic imaging system that provides improved ultrasonic images that maximize repeatability and reproducibility, especially in cardiac and cardiac-influenced imaging. A need also exists for an ultrasonic imaging system that provides a clear, easily diagnosable image that represents an accurate image through a cardiac cycle and minimizes adverse imaging effects such as respiration effects and movement effects.
The present invention synthesizes a cineloop of a compounded ultrasonic image, such as over a cardiac cycle, for example. In a real-time example, a series of image frames is recorded over a cardiac cycle and stored in an image array. A second series of image frames are recorded over a second cardiac cycle. The image frames of the second cardiac cycle are then temporally and spatially aligned with the image frames of the first cardiac cycle. The first and second series of image frames at then combined to form a series of synthesized image frames which are then stored in the image array in place of the first series of image frames. Subsequent series of image frames are also combined with the synthesized image frames to form new synthesized image frames. The series of image frames may be triggered to begin at a cardiac event such as the R-event. An age attribute may also be assigned to the image frames for use in weighting the image frame during combination. Additionally, a mismatch error estimate between image frames may be determined. Image frames or cardiac cycles with a high mismatch error may not be combined, or alternatively, only a portion of cardiac cycles having the least mismatch error may be combined. In addition, a cineloop may be constructed based around the standard deviation of the image frames to display non-repetitive variations in the cardiac cycle.
These and other features of the preferred embodiments of the present invention are discussed or apparent in the following detailed description of the preferred embodiments of the present invention.